This invention relates to containers, tanks or ships, for the storage or transportation of cryogenic liquids such as liquid natural gas (LNG), and is particularly concerned with containers, tanks or ships of the above type containing non-metallic, e.g. plastic, foam insulation and one or more liners, and preferably a low temperature resisting, i.e. low thermal expansion, liner such as nickel steel, and a support system for the liner or membrane, and for the foam insulation layers, and including a corner structure permitting transfer of loads from an inner liner to the tank wall and permitting movement of the insulation at such corner under such loads.
A container or tanker for the storage and/or transportation of a cryogenic liquid must be designed to withstand extremely cold temperatures. Generally vessels of this type are composed of an outer wall of a rigid structure, a heat insulating layer provided at the inside surface of such wall and an inner membrane on the inside surface of such heat insulating layer. Often several heat insulating layers of non-metallic, e.g. plastic, foam insulation, are employed and one or more membranes, particularly an inner liner or membrane such as a nickel steel liner in contact with the cryogenic liquid and one or more additional secondary liners positioned between foam insulation layers. The primary liner, generally made of a thin low temperature resistant (low thermal expansion) material such as nickel steel, is maintained in close contact with the surface of the adjacent heat insulating layer and transmits the internal pressure applied by the low temperature liquefied gases through the heat insulating layers to the outer container or the hull of a tanker. Illustrative of such a system is U.S. Pat. No. 3,814,275, to Lemons.
Of particular importance, the container or its insulation system must be capable of withstanding the thermal strains induced by the cold liquid and the transients during the cooling and warming cycles caused by the loading and unloading of the liquid, and the mechanically induced strains from the ship hull or container displacement during operation. The magnitude of such thermal strains induced when cooling most materials, in a restrained condition, to cryogenic temperatures is sufficient to cause yielding of the material, particularly the thin metal membranes employed as liners. This yielding will result in fracture and failure of such materials on subsequent warming and cooling cycles resulting in a failure of the system. Many designs and materials have been proposed to solve this problem. However, generally the solution to this problem has resulted in relatively complex membrane support systems which are often difficult to fabricate and are expensive.
Illustrative of the prior art in this respect are the following patents. U.S. Pat. No. 3,724,703 discloses a low temperature liquefied gas storage tank and tanker comprising a heat insulating layer of polyurethane foam, a thin liner, e.g. of nickel steel, and a vacuum pump to detect leaks. U.S. Pat. Nos. 3,931,424; 3,882,591; 3,319,431; 3,692,205; 3,694,986; 3,341,051 and 3,990,382 are illustrative of the prior art with respect to support systems for the foam insulation and the liner or membrane in cryogenic tanks. U.S. Pat. No. 3,399,800 discloses a corner support for the inner membrane of a liquefied gas tank in the form of rigid iron plates and thick metal brackets.
The liquefied natural gas containment system, e.g. described in above U.S. Pat. No. 3,814,275 may be installed on the ship hull or tank using many different installation sequences. Regardless of which installation sequence is selected, the final design may be divided into two categories, (1) the fiber reinforced foam insulation bonded directly to the ship hull or container wall, or (2) the fiber reinforced foam insulated bonded to a support structure which is bonded and/or bolted to the ship hull or container wall. The basic problems in installing the system by adhesive bonding in any large container are (1) cleaning the surface of the container for bonding, (2) controlling the temperature and humidity of the bonding area, and (3) having a relatively flat surface on which to bond the insulation.
To circumvent these problems one approach is to adhesive bond the foam insulation to support panels and bolt the panels in place to limit the adhesive bonding operations in the ship. In this method, however, the ship hull is not flat, and the insulation panels must be supported over a major part of their surface area. U.S. Pat. No. 3,698,588 discloses a load supporting thermal insulation and spacers disposed between the insulation and the outer wall of the container, but such spacers are of a rigid material such as metal.
Further, in the prior art foam insulation systems the primary membrane fits tight against the surface of the foam insulation. The cavity between the membrane (primary barrier) or liner and the container or ship hull is almost completely filled with the foam insulation (secondary barrier), and the cryogenic liquid gas will permeate into the foam insulation. During draining of the tank, this liquid, if allowed to change to a gas, will build up sufficient pressure behind the membrane to cause damage. The removal of the cryogenic liquid and/or gas from behind the primary barrier membrane after a leak occurs has been a major problem.